← Glossary
Cost basis
What you paid for the shares you still hold — the starting point for your gain or loss.
Your cost basis in a holding is the total you paid for the shares you still own, usually including the commissions on the buys. Your gain or loss is the holding's market value minus its cost basis.
When you sell part of a holding, part of the cost basis leaves with the shares sold. Which part depends on the method — average cost, or the oldest shares first — and tax rules may decide that for you.